Conventional equipment using linear vibration motors include vibration generators that inform of incoming calls by mechanical vibration, such as handy phones, compressors that compress and circulate gas or liquid, and reciprocating electric razors. In this specification, the handy phones refer to portable phones such as mobile phones and cellular phones. The compressors and the reciprocating electric razors use the linear vibration motors as their driving sources.
A typical linear vibration motor has a structure of a single-phase sync motor, i.e., it has a mover comprising a permanent magnet and a stator obtained by winding a coil around an iron core, and the mover reciprocates when an AC voltage is applied to the coil.
When generating vibrations by the reciprocating motion of the mover, a strong electromagnetic force is needed. However, an energy required for driving the linear vibration motor can be suppressed by forming a spring vibration system including the mover and a spring member supporting the mover. That is, in the linear vibration motor in which the mover is supported by the spring member, the spring vibration system including the mover is vibrated at its natural frequency (resonance frequency), whereby the linear vibration motor can be driven with a relatively low energy.
In the linear vibration motor, however, when the stroke length of the mover becomes larger than a predetermined allowable value, a problem such as a collision between the mover and the motor housing or breakage of the spring member may occur. Therefore, the position of the mover must be detected and controlled.
Japanese Published Patent Application No. Hei.11-324911 discloses a driving apparatus for driving a linear vibration motor, which includes a detector such as a position sensor for detecting the position of the mover of the linear vibration motor, and reduces the output of the linear vibration motor when the stroke length of the mover becomes larger than a predetermined allowable value, i.e., decreases the amplitude value of the voltage or current applied to the linear vibration motor, thereby preventing the linear vibration motor from being broken due to collision between the mover and the motor housing or extension of the spring member over a critical value.
As the above-mentioned position detector, there is employed a sensor that can detect the degree of displacement of the mover (the mover displacement amount) with respect to a mover reference position such as a mover neutral position without contacting the mover of the linear vibration motor. For example, a displacement gauge using an eddy current system, a displacement gauge using a differential transformer, or the like is employed.
However, when such sensor is employed, not only the production cost of the linear vibration motor is increased but also a space for mounting the sensor is needed, which leads to an increase in the size of the housing of the linear vibration motor. Further, when considering the compressor as an application of the linear vibration motor, such sensor may be used with being exposed to a high-temperature and high-pressure gas. Therefore, a problem on reliability of the sensor itself occurs, in other words, a sensor that can be reliably used in high-temperature and high-pressure atmospheres is required.
So, as a method for detecting the position of the piston, there is proposed a method of directly measuring the linear motor driving current and voltage which are supplied to the linear vibration motor, and deriving the position of the mover on the basis of the measured values, without using a position sensor placed in the linear vibration motor (refer to Japanese Unexamined Patent Publication No.Hei.8-508558).
Hereinafter, a description will be given of the mover position detection method used for a linear vibration motor, which is described in the above-mentioned application. The linear vibration motor described in this application is applied to a linear compressor. Therefore, this application describes a case where a mover that reciprocates within a cylinder so as to compress gas in the cylinder constituting the linear compressor is prevented from colliding against a cylinder head.
FIG. 11 is a diagram illustrating an equivalent circuit of a linear vibration motor for driving a mover.
In FIG. 11, L indicates an equivalent inductance [H] of a coil as a component of the linear vibration motor, and R indicates an equivalent resistor [Ω] of the coil. V indicates an instantaneous voltage [V] applied to the linear vibration motor, and I indicates a current [A] applied to the linear vibration motor. Further, α×v indicates a induced electromotive voltage [V] which is generated when the linear motor is driven, wherein α is a thrust constant [N/A] of the linear vibration motor, and v is an instantaneous velocity [m/s] of the linear vibration motor.
Here, the thrust constant α of the linear vibration motor indicates a force [N] which is generated when a unit current [A] is passed through the linear vibration motor. While the unit of the thrust constant α is expressed by [N/A], this unit is equivalent to [Wb/m] or [V·s/m].
The equivalent circuit shown in FIG. 11 is derived from Kirchhoff's law, and an instantaneous velocity v [m/s] of the linear vibration motor is obtained from the equivalent circuit.
That is, under the state where a driving voltage is applied to the linear vibration motor, the voltage (V) applied to the linear vibration motor is balanced with the sum of a dropped voltage (I×R) [V] due to the equivalent resistance of the coil of the linear vibration motor, a dropped voltage (L·dI/dt)[V] due to the equivalent inductance of the coil, and the induced electromotive voltage (α×v) [V] generated when driving the linear vibration motor, and then the following formula (1) is derived.
                    v        =                              1            α                    ⁢                      (                          V              -                              R                ×                I                            -                              L                ⁢                                                      ⅆ                    I                                                        ⅆ                    t                                                                        )                                              Formula        ⁢                                  ⁢                  (          1          )                    
The coefficients α[N/A], R[Ω], and L[H] used in formula (1) are constants unique to the motor, and these constants are known values. Accordingly, the instantaneous velocity v[m/s] can be obtained from these constants and the applied voltage V[V] and current I[A] which are measured, on the basis of formula (1).
Further, the mover displacement (a distance from an undefined reference position to the mover) x[m] is obtained by time integration of the instantaneous velocity v[m/s] as shown by the following formula (2). In formula (2), the constant Const. is a mover displacement at the start of the integration.x=∫vdt+Const.  Formula (2)
As described above, in the mover position detection method as proposed in the above application, the measured value V of the applied voltage and the measured value I of the applied current associated with the linear vibration motor are subjected to arithmetic processing including differentiation based on formula (1) to obtain the instantaneous velocity v of the mover, and further, the instantaneous velocity v is subjected to arithmetic processing including integration based on formula (2), whereby the mover displacement x can be obtained.
However, the mover displacement x obtained by the arithmetic processing based on formulae (1) and (2) is a displacement with respect to a certain position on the mover axis, and it is impossible to obtain a distance from the cylinder head which may be collided by the mover to the mover top dead point, directly from the displacement x.
To be more specific, when the compressor to which the linear vibration motor is applied is under a loaded condition, the mover center position (mover amplitude center position) in the mover reciprocating motion is offset with respect to the mover neutral position (i.e., the mover amplitude center position when the pressure in the compression chamber is equal to the back pressure) due to the pressure of a cooling medium gas, and the mover reciprocates around the offset mover amplitude center position. In other words, the mover displacement x obtained by formula (2) includes an average component.
However, every actual analog or digital integrator does not perform ideal integration processing for outputting a perfect response signal with respect to a constant or a DC input, but it is restricted in responding to a DC input. Therefore, an actual integrator cannot subject the mover displacement x to integration processing in which its average component is reflected. The reason why the DC response of the actual integrator is restricted is because the output of the integrator should be prevented from being saturated by unavoidable DC components in the input signal.
As a result, the mover displacement x[m] obtained by the integration processing based on formula (2) using the actual integrator is not a displacement from which an actual distance between the mover and the housing cannot be obtained directly, but a displacement simply indicating the mover position with reference to a certain point on the mover axis.
Therefore, the mover displacement x[m] obtained from formula (2) is converted into a mover displacement x′ indicating a mover position with respect to the mover amplitude center position. Further, using the converted mover displacement x′, arithmetic processing for obtaining a mover displacement xav″ with reference to the cylinder head, which indicates the mover amplitude center position, is carried out.
Hereinafter, these arithmetic processings will be described in more detail.
FIG. 12 is diagram schematically illustrating the mover position in the linear vibration motor housing (the cylinder in this case).
In this figure, Me denotes a mover, and Mc denotes an inner wall (cylinder inner surface) of the linear vibration motor housing that contains the mover.
Initially, three coordinate systems shown in FIG. 12, i.e., a first coordinate system X, a second coordinate system X′, and a third coordinate system X″, will be briefly described.
The first coordinate system X is a coordinate system expressing the mover displacement x and it has, as an origin (x=0), a certain point Paru on the mover axis. Accordingly, the absolute value of the displacement x indicates the distance from the point Paru to the mover front end position P.
The second coordinate system X′ is a coordinate system expressing the mover displacement x′ and it has, as an origin (x′=0), the mover amplitude center position Pav. Accordingly, the absolute value of the displacement x′ indicates the distance from the amplitude center position Pav to the mover front end position P.
The third coordinate system X″ is a coordinate system expressing the mover displacement x″ and it has, as an origin (x″=0), the cylinder head position Psh on the mover axis. Accordingly, the absolute value of the displacement x″ indicates the distance from the cylinder head position Psh to the mover front end position P.
Next, an arithmetic operation for obtaining the mover displacement x″ will be described.
A mover position (mover top dead point position) Ptd in which the mover is closest to the cylinder head is indicated by a displacement xtd on the first coordinate system X, and a mover position (mover bottom dead point position)Pbd in which the mover is farthest from the cylinder head is indicated by a displacement xbd on the first coordinate system X. Then, a mover stroke Lps[m] is obtained from a difference between the displacement xtd corresponding to the mover top dead point position Ptd on the first coordinate system X and the displacement xbd corresponding to the mover bottom dead point position Pbd on the first coordinate system X.
Further, the mover amplitude center position Pav in the state where the mover is reciprocating is a position which is apart from the displacement xtd of the mover position (mover top dead point position) Ptd in which the mover is the closest to the cylinder head, by a length (Lps/2) equal to half the mover stroke Lps[m], away from the cylinder head. Accordingly, the mover amplitude center position Pav is expressed by a displacement xav (=(xbd−xtd)/2) on the first coordinate system X.
Further, when the constant Const. in formula (2) is 0, a new function that indicates the mover front end position P by the mover displacement x′[m] is derived with the mover amplitude center position Pav as a reference (origin), in other words, on the second coordinate system X′.
Subsequently, a description will be given of a method for obtaining the mover displacement xav″ indicating a distance from the cylinder head position Psh to the mover amplitude center position Pav on the third coordinate system X″ with the cylinder head position Psh as an origin.
Under the state where the linear compressor draws in a cooling medium gas (suction state), i.e., under the state where the inlet valve is open, both of the pressure in the compression chamber and the pressure on the back of the mover are equal to the cooling medium inlet pressure. This is because the linear compressor is constructed so that the differential pressure becomes zero under the state where the inlet valve is open. In this state, a force from the pressure of the cooling medium that acts on the mover can be ignored. That is, in this state, the forces acting on the mover are only the repulsive force of the spring that is generated by bending of the support spring and the electromagnetic force that is generated by applying a current to the linear vibration motor. According to Newton's law of motion, the sum of these forces is equal to the product of the total mass of the movable member that is moving, and its acceleration.
Accordingly, under this state, the following formula (3) holds as an equation of motion relating to the movable member.m×a=a×I−k(x′+xav″−xini″)  Formula (3)
In formula (3), m is the total mass [kg] of the movable member that is reciprocating, a is the instantaneous acceleration [m/s/s] of the movable member, and k is the spring constant [N/m] of the support spring that is incorporated in the linear vibration motor. Further, xav″ is the above-mentioned displacement on the third coordinate system X″, which indicates the mover amplitude center position, and the absolute value of this displacement xav″ expresses the distance from the cylinder head position Psh to the mover amplitude center position Pav. Further, xini″ is the displacement on the third coordinate system X″, which indicates the mover neutral position Pini, and the absolute value of this displacement xini″ expresses the distance [m] between the mover neutral position (the position of the mover in the state where the support spring is not deformed) Pini and the cylinder head position Psh.
Here, the instantaneous acceleration a [m/s/s] is obtained as shown in the following formula (4), by differentiating the instantaneous velocity v[m/s] given by formula (1).
                    a        =                              ⅆ            v                                ⅆ            t                                              Formula        ⁢                                  ⁢                  (          4          )                    
Furthermore, the displacement x′[m] on the second coordinate system X′, which indicates the distance from the mover amplitude center position Pav to the mover front end position P, is obtained by setting the constant Const. in formula (2) at 0.
Furthermore, the total mass m[kg] of the movable member, the spring constant k[N/m] of the support spring, and the displacement xini″[m] on the third coordinate system X″, which indicates the distance from the cylinder head position Psh to the mover neutral position Pini, are known values, and the driving current I can be a measured value.
Accordingly, the displacement xav″ on the third coordinate system X″, which indicates the distance from the cylinder head position Psh to the mover amplitude center position Pav,. can be calculated using formula (3).
Further, the displacement xtd″[m] on the third coordinate system X″, which indicates the top dead point position of the mover (the position where the mover is closest to the cylinder head) Ptd, can be obtained as a displacement in a position which is apart from the displacement xav″ on the third coordinate system X″ obtained by formula (3) (the distance from the cylinder head position Psh to the mover amplitude center position Pav), by a distance equal to half the already-obtained mover stroke Lps[m] (=Lps/2), toward the cylinder head.
In this way, the mover stroke length Lps[m], and the displacement xtd″ on the third coordinate system X″, which indicates the mover top dead point position Ptd as a distance from the cylinder head position Psh, are calculated from the current I and the voltage V which are applied to the linear vibration motor.
Further, as an example of the method for detecting the mover position without using the position sensor, the inventors propose a method of using a mass/spring ratio m/k, without using the spring constant k (for example, referred to Japanese Published Patent Application No. 2002-354864).
However, in the above-mentioned method of deriving the mover position according to the position arithmetic operation based on the measured values of the driving current and the driving voltage of the linear vibration motor, the result of the arithmetic operation may include an error due to dispersion among units in the spring constant k or the mass/spring ratio m/k which are used in the arithmetic operation, their variations with time, changes caused by heat, and the like.
More specifically, when the spring constant k or the mass/spring ratio m/k α varies by 10%, the calculated mover absolute position varies by more than 10%. In such case, in order to avoid the collision between the mover and the cylinder head on the basis of the position of the mover calculated by the arithmetic operation using the above-mentioned formulae, a margin of 10% or more should be given to the clearance between the mover and the cylinder head. Accordingly, the stroke of the mover cannot be enlarged up to a position in which the mover approaches a collision critical position of the mover (i.e., a position where the mover contacts the cylinder head), which has been calculated by the arithmetic operation.
Further, when the mover reciprocates in such a manner that expansion and contraction of the support spring does not exceed an expansion/contraction range that is estimated for the support spring (estimated expansion/construction range), such reciprocating motion of the mover does not cause great variations with time. However, when the mover reciprocates in such a manner that the expansion and contraction of the support spring exceeds the estimated expansion/contraction range, as in the case of malfunction of the linear vibration motor, the spring constant k or the mass/spring ratio m/k may greatly vary.
In such cases, the linear vibration motor must be replaced together with the motor driving apparatus, which leads to deterioration in the reliability of the linear vibration motor as a driving apparatus.
It is also possible to make the support spring larger to prevent the expansion and contraction of the support spring from exceeding the estimated range even in the case of malfunction of the linear vibration motor. However, by doing so, not only the outer dimension of the linear vibration motor is increased but also the production cost is increased.